transfer: Fix potential NULL pointer dereference

Commit 0ef54abf52 accidentally used the conn variable before the
assertion for it being NULL. Fix by moving the assignment which use
conn to after the assertion.

Closes: #8857
Reviewed-by: Daniel Stenberg <daniel@haxx.se>

lib/transfer.c

@@ -1896,11 +1896,14 @@ Curl_setup_transfer(
   struct SingleRequest *k = &data->req;
   struct connectdata *conn = data->conn;
   struct HTTP *http = data->req.p.http;
-  bool httpsending = ((conn->handler->protocol&PROTO_FAMILY_HTTP) &&
-                      (http->sending == HTTPSEND_REQUEST));
+  bool httpsending;
+
   DEBUGASSERT(conn != NULL);
   DEBUGASSERT((sockindex <= 1) && (sockindex >= -1));
 
+  httpsending = ((conn->handler->protocol&PROTO_FAMILY_HTTP) &&
+                 (http->sending == HTTPSEND_REQUEST));
+
   if(conn->bits.multiplex || conn->httpversion == 20 || httpsending) {
     /* when multiplexing, the read/write sockets need to be the same! */
     conn->sockfd = sockindex == -1 ?